Ultrasonic generator and burner

ABSTRACT

An oscillator drives an ultrasonic transducer by rectangular waveform voltage. An automatic resonant frequency tracking system is provided in which the driving voltage is positive-fed back to the oscillator, and a system for controlling the amplitude of oscillation is provided by the negative-feed back of the driving voltage to a DC power source for the oscillator.

United States Patent [1 1 Fujieda a al.

ULTRASONIC GENERATOR AND BURNER Inventors: Hiroshi Fujieda, Yamatokoriyama;

Taro Yamamoto, Uji, both of Japan Assignee: Matsushita Electric Industrial Co.,

Ltd., Osaka, Japan Filed: July 23, 1973 Appl. N0.: 381,786

Foreign Application Priority Data July 31, 1972 US. Cl. 31/1; 239/102; 318/118 Int. Cl 23c 3/02; F230 11/00 Field of Search 431/1; 239/4, 102;

References Cited UNITED STATES PATENTS 2/1964 Wilson 318/118 Japan 47-77218 Primary Examiner-Charles J. Myhre Assistant ExaminerWilliam C. Anderson 57 ABSTRACT An oscillator drives an ultrasonic transducer by rectangular waveform voltage. An automatic resonant frequency tracking system is provided in which the driving voltage is positive-fed back to the oscillator, and a system for controlling the amplitude of oscillation is provided by the negative-feed back of the driving volt age to a DC power source for the oscillator.

22 Claims, 4 Drawing Figures LINE VOLTAGE 601 602 {L603 l0l\ 6 EPROTECTIVE RELAY POST PURGE RELAY \5 2 20 1 l,2| K 183: 0.0 POWER IGNITION I05o SOURCE 3 TRANSFORMER .AND OSCILLATOR g 202 10 I2 2 l l 1 IL 203 I 1 3 1 403 FUEL 80 b, 402 SUPPLY SOLENOID? l VALVE 9| |o0 11 2 00 -CONTR0L :82 I02 CIRCUIT I05 SHEET 1 LINE VOLTAGE G- 1 |o||o2 TT l2 |o|| |oo CONTROL 3' CIRCUIT PLSOLENOID -I m VALVE r Wcmm 22 T\Q [O 30c VOLTAGEREGULATOR' l a O 20 2| LINE VOLTAGE I ULTRASONIC GENERATOR AND BURNER BACKGROUND OF THE INVENTION Ultrasonic energy has found wide applications in many fields such as cleaning, welding, atomization of liquids and so on. In general, magnetostrictive or piezoelectric devices are used to convert electrical oscillation into mechanical oscillation. Furthermore, a horn is attached to an ultrasonic transducer in order to amplify the mechanical oscillation, and is used in ultrasonic welding machines or liquid atomization devices.

Ultrasonic liquid atomization devices have been for example used in conjunction with liquid fuel burners because of the advantages to be described hereinafter over liquid fuel atomization method of the type in which liquid fuel is injected through a small orifice under high pressure. A first advantage is that the ultrasonic liquid fuel atomization device can eliminate a motor-driven pump for injecting liquid fuel under high pressure through an orifice because a thin film formed upon an oscillating surface of an ultrasonic transducer may be torn off into atomized particles. Therefore, liquid fuel may be supplied to the oscillating surface by, for example, gravity feed means, so that no pump is required.

A second advantage is that the ultrasonic liquid atomization device may use a relatively large diameter nozzle in order to supply liquid to be atomized to the oscillating or atomization surface. In the conventional methods, liquid fuel must be discharged through a small orifice so that clogging tends to occur very often, and this problem is overcome by the ultrasonic fuel atomization method.

A third advantage is that the quantity of atomized liquid fuel may be continuously varied over a wide range from zero to the maximum or vice versa only by controlling the flow rate of liquid fuel to be supplied to the oscillating or atomizing surface as compared with the conventional method in which the quantity of atomized liquid fuel may be varied only over a limited range.

However, in order to make full use of the above advantages of the ultrasonic liquid fuel atomization method the following problems or demands must be solved or satisfied.

A first problem is to drive an ultrasonic transducer at a resonant frequency thereof or at a frequency very close thereto in order to attain high efficiency in operation because the ultrasonic transducer has generally a high Q. Theresonant frequency is generally dependent upon the dimensions, configurations and so on so that when the dimensions of the ultrasonic transducer are varied in response to the temperature change the reso- 'nant frequency is also varied. Therefore the ultrasonic oscillator must drive the ultrasonic transducer at its resonant frequency or at a frequency very close thereto.

A second problem is to maintain an optimum amplitude of oscillation in order to obtain desired particles sizes of atomized fuel. When the amplitude is increased in excess of an optimum level, the particle size is gradually increased due to cavitation whereas when the amplitude is decreased below a optimum level, no atomization of liquid fuel occurs.

A third problem is a provision of means for applying a relatively large transitional input to the ultrasonic transducer when atomization of liquid fuel is started and then applying a steady-state input to the ultrasonic transducer a predetermined time after the start of the atomization. More particularly, when a thin film of liquid fuel is formed upon the atomizing surface and has not atomized yet, greater energy than when liquid fuel film is being atomized is required for atomization. This means that the load of the ultrasonic transducer is greater when the atomization is started so that even when the steady-state input is applied the amplitude of oscillation is less than that in the steady-state. In summary, the initial input to the ultrasonic transducer must be increased higher than the input required for the atomization in the steady -state and then returned to the normal input level after the start of the atomization. In this specification, this requirement is referred to as the hysteresis for the sake of convenience in description.

A fourth problem is to provide means for immediately stopping the atomization and simultaneously preventing liquid fuel from being discharged without being atomized when the breakdown of any of the component parts of the ultrasonic liquid fuel atomizing device occurs. Especially when the ultrasonic liquid fuel burners are not provided with such means, the breakdown of the ultrasonic transducer is inevitable.

A fifth problem is to increase the amplitude of oscillation of the ultrasonic transducer when the temperature of liquid to be atomized drops because the viscosity of liquid is increased.

A sixth problem is to provide means for immediately causing liquid fuel supply control means to interrupt the liquid fuel supply when the breakdown of a motor for driving a blower occurs because the liquid fuel supply control means and the blower are controlled independently of each other in operation. If the supply of liquid fuel to the ultrasonic transducer is not immediately stopped, the atomization of liquid fuel would be continued, but the incomplete combustion inevitably would occur because of the shortage of primary air. Furthermore, not only the ultrasonic transducer but also ignition means would be damaged seriously.

A seventh problem is to provide means for discharging liquid fuel oil remaining in fuel supply means after said fuel supply control means such as a solenoid valve is closed so that the remaining liquid fuel which is not atomized may be prevented from being discharged into a combustion chamber or from remaining within a housing of an ultrasonic liquid fuel atomizing device.

SUMMARY OF THE INVENTION In view of the above, a first object of the present invention is to provide an ultrasonic liquid atomizing device which may substantially overcome the above problems.

A second object of the present invention is to provide an ultrasonic liquid fuel burner utilizing the above ultrasonic liquid atomizing device, thereby satisfying the above demands and overcoming the above problems.

When a damped admittance of an ultrasonic trans ducer is sufficiently so small that it is negligible as compared with a motional admittance, the latter becomes maximum at a resonant frequency of the ultrasonic transducer when it is driven at a constant voltage, so that the driving current also becomes maximum. The positive feedback of the driving current is therefore utilized to provide an oscillator of the type capable of automatically tracking a resonent frequency. The negative feedback of the driving current is also utilized in order to control'the output voltage of the oscillator thereby stabilizing a desired amplitude of oscillation. According to the present invention, the oscillator and arrangement of the types described above are combined in a liquid atomizing device or liquid fuel burner so that reliability and safety in operation may be greatly insured.

The above and other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagram of an ultrasonic liquid atomizing device according to the present invention;

FIG. 2 is a diagram of a two-output DC power source, a switching circuit and a control circuit for controlling the switching circuit;

FIG. 3 is a diagram illustrating an ultrasonic liquid fuel burner in accordance with the present invention; and

FIG. 4 is a circuit diagram of a timer incorporated in the burner shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The ultrasonic transducer used in the invention is illustratedin FIG. 1. In FIG. 1, a magnetostrictive device has a coil 11 mounted on its legs 10a and has its oscillating surface attached to the bottom of a frustoconical horn 12. When AC voltage whose frequency is equal to a resonant frequency of the ultrasonic transducer is impressed across the coil 11, its motional admittance becomes maximum, so that the maximum current flows therethrough. As a result, the magnetostrictive device 10 may transform the electrical oscillations into the mechanical oscillation with the maximum efficiency, and the amplitude of mechanical oscillation perpendicular to an end surface 13 of the frustoconical horn 12 may be amplified greater than the amplitude of the mechanical oscillations at the bottom surface of the horn 12. The amplification factor is dependent upon the material, configuration, and so on of the horn 12.

When the ultrasonic transducer of the type described above is used for atomizing liquid, means for supplying liquid to be atomized to the end surface 13 of the horn 12 is provided. For example, as shown in FIG. 1, the discharge port of a nozzle 14 is opened at the end surface 13 of the horn 12 and the inlet port thereof is connected to a liquid feed line at the node of the horn 12.

Next, an oscillator for driving the ultrasonic transducer 10 will be described. An amplifier generally indicated by P comprises a phase shift circuit which in turn comprises a capacitor 40 and a variable resistor 41, and a two-stage amplifier. The first stage of the amplifier comprises a blocking capacitor 42, bias resistors 43, 44 and 46, a bypass capacitor 47 of the resistor 46, a transistor 45, and the primary winding 49 of a coupling transformer 48, and the second stage comprises the secondary windings 50 and 51 of the transformer 48, two transistors 52 and 53 and a blocking capacitor 54 inserted in series in an output circuit. A feedback circuit comprises a current transformer 60 whose primary winding 61 is connected in series to the output circuit of the amplifier circuit, and a resistor 63 inserted in parallel with the secondary winding 62 of the transformer 61. The voltage across the primary winding 61 is fed back to the amplifier circuit P, which forms an oscillator together with the feedback circuit.

Next, the mode of operation of the oscillator will be described. Direct current is supplied to the oscillator from a DC power source to be described hereinafter, and the transistors 52 and 53 are alternately turned on and off so that substantially rectangular waveform voltage is derived across the coil ll. The peak voltage is equal to one half of the DC voltage of the DC voltage source. Assume that a damped admittance across the terminals of the ultrasonic transducer be sufficiently small as compared with the motional admittance at a resonant frequency and that a frequency of the rectangular waveform voltage be equal to a resonant frequency f of the ultrasonic transducer. As described hereinbefore, the motional admittance becomes maximum at the resonant frequency f and becomes lower as the frequency is deviated from the resonant frequency f The rectangular waveform voltage applied across the ultrasonic transducer may be expanded as follows:

where e is voltage,

E is peak voltage equal to one half of DC voltage E m is angular frequency (equal to 2'n'f and t is time, The peak voltages of the odd-number harmonics are l/3, l/5,. and l/(2n-l) of the peak voltage of the fundamental component. Therefore, the admittance of the ultrasonic transducer is low with respect to the frequencies of the high frequency components and the peak voltages of the harmonics are decreased as the order of the harmonics is increased so that the current whose frequency is equal to the resonant frequency f flows through the coil 11. From Eq. (1), the fundamental harmonic component e,, which determines the mode of operation of the ultrasonic transducer, is given y e, (4/11') (E sin 21rf t) It is seen that the fundamental harmonic component e, is only in proportion to DC power voltage and is not dependent upon admittance of the ultrasonic transducer. Therefore, it serves as a constant voltage source for driving the ultrasonic transducer with a constant voltage. Thus, the constant voltage source is provided simply by alternately switching the transistors 52 and 53. Since the transistors 52 and 53 are used for switching operation, the collector loss may be considerably decreased, and the efficient DC-AC conversion may be attained. As seen from BO. (2), the amplitude of the fundamental harmonic component 2 is dependent only upon the DC voltage source so that the voltage applied across the ultrasonic transducer and hence the level of the driving current may be easily varied by changing the DC source voltage. The current in proportion to the current flowing through the primary winding 61 of the transformer flows through the resistor 63 connected in parallel with the secondary winding 62 thereof, so

that a voltage across the resistor 63 is in proportion to the driving current with the frequency f On the other hand, the damped admittance of the ultrasonic transducer is sufficiently smaller as compared with its motional admittance and the ultrasonic transducer is driven by the constant voltage so that the current flowing through the coil 11 is in proportion to the oscillation of the ultrasonic transducer. As a result, the oscillation of the ultrasonic transducer becomes maximum at the resonant frequency f and the current flowing through the coil 11 also becomes maximum. Furthermore, the voltage across the primary winding 61 of the transformer 60 becomes maximum. The voltage across the primary winding 61 is positive-fed back to the phase shift circuit and the output of the phase shift circuit is amplified by the transistor 45, so that the switching action of the transistors 52 and 53 may be continued. The constant of the phase shift circuit is controlled by the variable resistor 41 so that the overall phase shift becomes zero or 360 at f Furthermore, the amplification factor higher than unit is selected. Thus, the automatic oscillation at the resonant frequency f may be attained.

The commercial line voltage impressed across a pair of input terminals and 22 is dropped to a suitable voltage by a transformer 22, rectified by a bridge rectifier 23 and smoothed by a smoothing capacitor 24. The voltage across the smoothing capacitor 24 is an input current to a DC voltage regulator which comprises a transistor 34, another transistor 35 for detecting and amplifying error, a zener diode 32 for supplying a reference voltage, bias resistors 31 and 33 and a capacitor 36. The AC voltage across the resistor 63 in the feedback circuit is in proportion to the driving current flowing through the coil 11 and hence the oscillation amplitude of the ultrasonic transducer. This voltage is rectified and smoothed by a rectifier circuit comprising a diode 64, a resistor 65 and a capacitor 66, and is substantially in proportion to the driving current flowing through the coil 11. Therefore, the voltage across the capacitor 66 is in proportion to the oscillation amplitude of ultrasonic transducer and is used to regulate the output voltage of the oscillator so that the oscillation of the ultrasonic transducer may be maintained constant. The voltage across the capacitor 66 is divided by a series circuit comprising a resistor 68 and a variable resistor 67 and then applied to the base ofv the transistor 35,

so that the driving current may be held at a predetermined level. As a result, the oscillation of the ultrasonic transducer may be maintained at a predetermined constant level.

Next, the mode of operation for controlling the oscillation of the ultrasonic transducer will be described. When the speed of oscillation of the ultrasonic transducer becomes higher than a predetermined level, the driving current is increased so that the AC voltage across the resistor 63 in parallel with the secondary 62 of the transformer 60 will be also increased in proportion. As a result, the DC voltage across the capacitor 66 is also increased so that the input to the voltage regulator S is also increased. That is, the voltage between the base and emitter of the transistor 35 is increased whereas the voltage across the collector and emitter thereof is dropped so that the output voltage of the DC voltage regulator S is dropped. As a result, the oscillation speed of the ultrasonic transducer is reduced. The above step may be cycled until the speed of oscillation of the ultrasonic transducer is stabilized to a constant speed which corresponds to a voltage predetermined by the variable resistor 67. In like manner, when the speed of oscillation of the ultrasonic transducer becomes lower than a predetermined level, it is restored to a predetermined speed. Thus the speed of the ultrasonic transducer may be always maintained at a predetermined level.

Since the amplitude of oscillation of the ultrasonic transducer is expressed by dividing the speed by the angular frequency, the amplitude may be maintained constant when the angular frequency deviation is negligible.

For example, when the magnetostrictive device 10 is made of a ferrite, is of the 1r type and oscillates at 28 KHz and the horn 12 is made of an aluminum and is of exponential type, the resonant frequency deviation is 500 Hz when the temperature is varied from 20C to +C. However, the frequency deviation of 500 Hz is negligible since the resonant frequency is almost equal to 28 KHz. Therefore, the amplitude may be also stabilized over the above temperature range.

When the ultrasonic transducer, the oscillator, the DC voltage regulator and the DC voltage source are used in conjunction with an ultrasonic liquid atomizing device, the electrical input must be increased higher than the steady current when the liquid atomizing device is started. In this case, the hysteresis problem may be overcome by the following two methods. The first method will be described with reference to FIG. 2. In the arrangement shown in FIG. 2, a two-input DC voltage source is used as a DC power source for the ultrasonic oscillator, and a switching circuit for selecting one of the two outputs and a control circuit for controlling this switching circuit are provided. The commercial line voltage which is impressed across the pair of input terminals 20 and 21 is dropped by the transformer 22 to a suitable voltage, rectified by the bridge rectifier 23 and smoothed by the smoothing capacitor 24. A switching circuit comprises a relay with a movable contact 122 and stationary contacts 122A, 122B and 122C and a capacitor 29. One DC output is derived directly whereas the other DC output is derived through a resistor 25. That is, when the normally opened contact 12213 is closed, the high voltage is applied to the oscillator whereas when the low voltage is normally supplied to the oscillator through the nor mally closed contact 122A connected to the resistor 25. The control circuit for controlling the switching circuit comprises a coil 121 of the relay 120, a switching transistor 26, a capacitor 27 and a resistor 28. When the line voltage is applied to the input terminals 20 and 21, the current flows into the base of the transistor 26 through the capacitor 27 and 28 so that the transistor 26 is turned on. As a result, the relay 120 is energized so that the normally opened contact 1223 is connected to the common contact 122C to apply the higher voltage to the ultrasonic oscillator. Since the output of the oscillator is dependent upon the voltage of the DC power source, the output is increased as the output voltage of the DC power source is increased. The base current of the transistor 26 is exponentially decreased and so is the collector current. Finally, the relay 120 is de-energized so that the normally closed contact 122A is closed again. The lower voltage is therefore applied to the ultrasonic generator through the resistor 25, so that the output of the ultrasonic generator is also decreased. It is very simple to set the voltage across the capacitor 24 to such a level that liquid may be sufficiently atomized and to select the value of the resistor 25 in such a manner that the steady output may be supplied to the ultrasonic generator.

Next, referring back to FIG. 1, the second method will be described. When the control input voltage of the voltage regulator is zero when the atomization of liquid is started, the voltage across the emitter and collector of the transistor 34 is very low so that the output voltage of the voltage regulator becomes maximum. Therefore, when the atomization is started the control input voltage of the voltage regulator is made zero and thereafter it must be increased to the 100 percent level. That is, a timer is required to increase the control input voltage from zero to 100 percent within a predetermined time. This arrangement will be described with reference to FIG. 1. A transistor 71 is connected in parallel with the resistor 68, and has its base connected to the emitter of the transistor 34 through a resistor 72 and a capacitor 73 connected in series. Initially, the capacitor 73 is short-circuited so that the base current of the transistor 71 is high so that the transistor 71 is turned on. As a result, the control input voltage of the voltage regulator drops. The values of the variable resistor 67 and resistor 68 are so selected that the decrease in control input voltage is less than the breakdown voltage of the zener diode 32. Therefore, the output voltage becomes zero in practice. As time elapses, the collector current of the transistor 71 is reduced to zero so that the control input voltage is increased to a 100 percent level.

Next, the arrangement for counteracting the case, when the breakdown of the magnetostrictive device occurs so that the liquid is not atomized and flows along the oscillating surface 13 of the horn 12, will be described hereinafter. A solenoid valve 92 is inserted between sections of pipe 90 and 91 for feeding liquid to the liquid atomizing horn 12, and is controlled by a control circuit 100 in response to the variation in frequency or amplitude of the ultrasonic transducer 10, that is the driving current flowing through the coil 11 thereof. That is, when the amplitude of oscillation is in excess of a predetermined level, the solenoid valve 92 is opened so that the liquid is supplied, but when the amplitude becomes lower than a predetermined level the solenoid valve 92 is closed to interrupt the supply of the liquid fuel to the atomizing horn 12. For example, the control circuit 100 comprises a relay and a Schmitt circuit interconnected in such a manner so that when the amplitude becomes lower than a predetermined level, the relay is energized so that the supply of current from the feed line through terminals 101 and 102 to the solenoid valve 92 is cut off. The DC voltage across the capacitor 24 may be supplied to the Schmitt circuit (not shown) through a pair of input terminals 103 and 105 whereas the input voltage or signal is applied to the terminal 104 of the Schmitt circuit from the capacitor 66. When the amplitude of oscillation of the ultrasonic transducer becomes less than a predetermined level for any cause, the driving current flowing through the coil 11 is decreased so that the voltage across the capacitor 66 is also decreased. Then, the Schmitt circuit is triggered to energize the relay so as to interrupt the current supply to the solenoid valve 92, thereby interrupting the fuel supply to the horn 12.

When the ultrasonic liquid atomizing device is used over a wide temperature range, the viscosity of liquid to be atomized is increased as the temperature drops. It is therefore very advantageous to provide means for automatically increasing the amplitude of oscillation of the liquid atomizing horn 12 in response to the temperature drop. For this purpose, a thermistor is positioned within the liquid supply line so as to detect the temperature of the liquid. The thermistor 80 is connected to the variable resistor 67 so that the contol input voltage applied to the DC voltage regulator is dependent upon the variation in resistance of the thermistor 80 in response to the temperature variation. As a result, the output voltage of the DC voltage regulator is varied in response to the temperature change so that the amplitude of oscillation of the oscillating surface 13 of the horn 12 may be varied in response to the temperature variation. That is, when the temperature of liquid supplied drops, the control input voltage to be applied to the DC voltage regulator also drops so that the output voltage of the DC voltage regulator rises. As a result, the output voltage of the ultrasonic generator rises so that the amplitude of oscillation at the oscillating surface 13 of the horn 12 is increased. Thus, the liquid may be atomized to a predetermined degree.

Next, the application of the fuel atomizing device of the type described above to the ultrasonic liquid fuel burner will be described hereinafter with reference to FIG. 3, in which reference numeral 1 denotes a section generally indicated by 1 in FIG. 1 including the oscillator and the DC power source for supplying DC to the ultrasonic generator. An ignition transformer 2 is provided in order to produce a spark between the oscillating surface 203 and an ignition plug 202 supported by an insulator. Combustion air is supplied into a housing 301 by a blower 3 driven by a motor. A photocell or photoelectric transducer means 401 is disposed within the housing 301 in such a manner that it may intercept the light from the flame through an opening 403 when combustion air is flowing through the housing 301, but it may not intercept the light by a shield member 402 when combustion air is not supplied into the housing 301. Within the housing 301 is also disposed the ultrasonic liquid atomizer comprising the ultrasonic transducer, the fuel supply pipe, the ignition plug and the blower and the photoelectric transducer means 401. Air flows within the housing 301 around the ultrasonic transducer 10 and the atomizing horn 12 and is discharged through openings 302 and 403 into a combustion chamber. A protective relay 6 contols, in response to the signal from the photoelectric transducer means 401, the ignition transformer 2, the control circuit of the solenoid valve 92 and a post purge timer 5 to be described hereinafter. Power is supplied through a pair of terminals 601 and 602 and a switch 503 to the relay 6. The post purge timer 5 is adapted to control the ultrasonic generator and the DC power source 1 and the motor for driving the blower 3.

Next, the mode of operation will be described hereinafter. When the switch 603 is closed, power is supplied to the ignition transformer 2, the post purge timer 5 and the control circuit 100 through the protective relay 6 and also to the driving device 1 including the ultrasonic oscillator and the DC power source and the blower motor. The driving current flows through the coil 11 of the ultrasonic transducer 10 to cause the oscillations, and the combustion air is supplied by the blower 3 through the housing 301 into the combustion chamber. The solenoid valve 92 is energized to open itself to supply the liquid fuel to the nozzle 14 of the horn 12 through the pipe lines 91 and 90. Liquid fuel is atomized at the end or oscillating surface 13 of the horn l2 and mixed with primary air supplied by the blower 3, and the fuel-air mixture is ignited by the spark generated between the ignition plug 202 and the edge 203 of the end surface 13 of the horn. After a predetermined time interval after the ignition, a timer incorporated within the protective relay 6 interrupts the power supply to the ignition transformer 2, and the steady state combustion is started. The shield member 402 is in the position indicated by the dotted lines in FIG. 3 because primary air is supplied so that the photoelectric transducer means 401 intercepts the light from the flame through the opening 403 and gives the signal to the protect relay 6.

To stop the combustion, the switch 603 is opened so that the power supply to the solenoid valve 92 is interrupted. As a result, liquid fuel supply to the horn 12 is also interrupted. The power supply from the protect relay 6 to the post purge timer 5 through a pair of terminals E and F is interrupted, but the power is supplied to the post purge timer 5 from the terminal 602 so that the power supply to the blower motor and to the driving device 1 is maintained for a predetermined time through the terminals 20 and 21. That is, the post purge timer 5 functions as an off-delay timer. Since the power is supplied to the blower moter and to the driving device 1 in the manner described above, the liquid fuel remaining in the pipe line 90 is burnt or atomized and discharged into the combustion chamber so that no liquid fuel remains within the housing 301.

In case of the breakdown of the blower 3 so that no primary air is supplied into the housing 301, the shield member 402 moves to the position indicated by the solid lines in FIG. 3 so that no light is intercepted by the photoelectric transducer means 401. As a result the protective relay 6 interrupts the power supply to the solenoid valve 92 and to the post purge timer 5, so that the combustion is stopped. Thus, the shield member 402 functions as means for detecting whether primary air is supplied or not.

When the driving current flowing through the coil 1 1 drops below a predetermined level, the protective relay 6 interrupts the power supply from the protective relay 6 to the solenoid valve 92 so that the solenoid valve 92 is closed. As a result, the combustion is stopped, the photoelectric transducer means 401 does not detect the light from combustion flames and the protective relay 6 functions to interrupt the power supply to the solenoid valve 92 and to the post purge timer 5.

Next, referring to FIG. 4, the post purge timer 5 which functions as off-delay timer means will be described in more detail hereinafter. Commercial voltage applied to a terminal F is rectified and smoothed by a series-connected circuit comprising a diode 501 and a capacitor 502, and the voltage across the capacitor 502 is applied through a resistor 503 to the base of a switching transistor 504 whose load is a coil 505 of a relay, A normally closed contact 505A of the relay is connected to the terminal F; a normally open contact 5058 is connected to a terminal 602; and a common contact 505C is connected to the terminal 21. The voltage applied to the terminal 21 is rectified and smoothed by a series-connected circuit comprising a diode 506, a resistor 507 and a capacitor 509, and the voltage across the capacitor 509 is applied to the relay coil 505, the transistor 504 and a resistor 508.

When the switch 603 (See FIG. 3) is closed, the commercial power is supplied so that the base current flows into the transistor 504. The transistor 504 is turned on to energize the relay so that the movable contact 505D interconnets the normally opened contact 505B and the common contact 505C. As a result, the power supply through the terminal F is interrupted, and the capacitor 502 is dischared through the base of the transistor 504 and the resistor 503. The discharging current is decreased in time expotentially so that the collector current of the transistor 504 which flows through the relay coil 505 is also reduced. As a result, the relay is de-energized so that the movable contact 505D interconnects the normally closed contact 505A and the common contact 505C. However, no power is applied to the terminal F so that no power is supplied from the terminal 21. Thus, the power supply through the terminal 21 is interrupted a predetermined time interval after the interruption of power supply from the protective relay 6 through the terminal F.

As described hereinbefore, according to the present invention, the ultrasonic transducer whose Q is high and damped admittance is sufficiently small as compared with the motional admittance at a resonant frequency is driven always at its resonant frequency in a very efficient manner. Since only the positive feedback circuit is provided, in such a manner that the driving current flowing through the coil is converted into the voltage which in turn is fed back to the amplifier circuit, the circuit is very simple in construction. Furthermore, the voltage regulator maintains a predetermined constant voltage so that the oscillation of the ultrasonic transducer may be maintained constant by the negative feedback of the driving current. The voltage regulator also serves to compensate the fluctuation of the commercial or line voltage so that the constant oscillation of the ultrasonic transducer may be further ensured. Moreover, the very effective andautomatic liquid atomization can be effected when a simple additional circuit is added, and a very safe ultrasonic liquid fuel burner may be provided when a simple mechanism and circuit are added.

So far, the transducer has been described as being a magnetostrictive device, but it will be understood that the present invention is also applied to a piezoelectric device because, according to the presnet invention, it is not required to provide a coil for detecting oscillation in addition to the driving coil.

What is claimed is:

1. An ultrasonic generator comprising in combination a. an ultrasonic transducer whose motional admittance becomes maximum at a resonant frequency,

b. an oscillator comprising an amplifier driving said ultrasonic transducer with a square waveform voltage at a frequency substantially equal to said resonant frequency, whereby a surface of the transducer oscillates and a feedback circuit for feeding back to said amplifier an AC control voltage in proportion to the driving current which drives said ultrasonic transducer,

0. a DC power source for supplying DC power to said oscillatonsaid DC power source being provided with a DCvoltage regulator and (1. means for converting said AC control voltage into a DC control input voltage for said regulator.

2. An ultrasonic atomizer comprising an ultrasonic generator as defined in claim 1 further comprising means for supplying liquid to be atomized to the oscillating surface of said ultrasonic transducer.

3. An ultrasonic atomizer comprising an ultrasonic generator as defined in claim 1 wherein said DC power source comprising means for selectively supplying two different output voltages.

4. An ultrasonic atomizer as defined in claim 2 wherein said converting means comprises a switching circuit for selectively supplying one of two different output voltages, and

a control circuit for controlling said switching circuit.

5. An ultrasonic atomizer as defined in claim 4 further comprising means responsive to said driving current for controlling the flow rate of liquid to be supplied to said ultrasonic transducer.

6. An ultrasonic atomizer as defined in claim 4 further comprising means responsive to said driving current for controlling the flow rate of liquid to be supplied to said ultrasonic transducer. 7

7. An ultrasonic atomizer as defined in claim 2 further comprising timer means connected to said DC voltage regulator for increasing said control input voltage from percent to 100 percent within a predetermined time interval.

8. An ultrasonic generator as defined in claim 7 further comprising means responsive to said driving current for controlling the flow rate of liquid to be supplied to said ultrasonic transducer.

9. An ultrasonic atomizer as defined in claim 7 further comprising means for detecting the temperature of liquid to be atomized and for varying said control input voltage in response to the detected temperature.

10. An ultrasonic atomizer comprising in combination a. an ultrasonic transducer whose motional admittance becomes maximum at a resonant frequency,

b. an oscillator comprising an amplifier driving said ultrasonic transducer with a square waveform voltage at a frequency substantially equal to said resonant frequency, wherebythe surface of the transducer oscillates and a feedback circuit for feeding back to said amplifier an AC control voltage in proportion to the driving current which drives said ultrasonic transducer,

c. a DC power source supplying DC power to said oscillator,

d. liquid to be atomized to the oscillating surface of said ultrasonic transducer and e. means responsive to said driving current for con-.

trolling the flow rate of liquid to be supplied to said ultrasonic transducer.

11. An ultrasonic liquid fuel burner comprising A an ultrasonic generator comprising a. an ultrasonic transducer whose motional admittance is maximum at a resonant frequency,

b. an oscillator comprising an amplifier adapted to drive said ultrasonic transducer with a square waveform voltage at a frequency substantially equal to said resonant frequency, and a feedback circuit for feeding back to said amplifier an AC voltage in proportion to the driving current which drives said ultrasonic transducer,

c. A DC power source for supplying DC power to said oscillator,

d. means for supplying liquid fuel to be atomized to an oscillating surface of said ultrasonic transducer,

e. a DC power source providing two different output voltages,

f. a switching circuit means for selecting one of said two output voltages of said DC power source, and

g. a control circuit for controlling said switching circuit;

B. an ignition electrode for igniting the atomized liquid fuel,

C. a combustion chamber,

D. a motor driven blower for discharging atomized liquid fuel,

E. housing means surrounding an atomizing section including said ignition electrode and said motordriven blower for causing air to flow along said atomizing section,

F. photoelectric transducer means disposed within said housing means for providing a combustion signal in response to light emitted from the combustion flame in said combustion chamber,

G. means for controlling the supply of liquid fuel to be supplied to said ultrasonic transducer, and

H. power supply control means connected to said DC power source, said ignition electrode, the motor for driving said blower and said liquid fuel supply control means for interrupting the operation of the apparatus connected thereto in response to an absence of the signal from said photoelectric transducer means.

12. An ultrasonic liquid fuel burner as defined in claim 11 further comrising means for detecting whether air is flowing through said housing means, said power supply control means being adapted to be actuated in response to the signals from said air flow detecting means.

13. An ultrasonic liquid fuel burner as defined in claim 11 further comprising means responsive to said driving current for controlling said liquid fuel supply control means.

14. An ultrasonic liquid fuel burner as defined in claim 11 further comprising timer means for delaying the operation of said DC power source and said motor of said blower.

15. An ultrasonic liquid fuel burner comprising A. an ultrasonic generator comprising a. an ultrasonic transducer whose motional admittance is maximum at a resonant frequency,

b. an oscillator comprising an amplifier means for driving said ultrasonic transducer with a square waveform voltage at a frequency substantially equal to said resonant frequency whereby a surface of said transducer oscillates, and a feedback circuit for feeding back to said amplifier an AC voltage in proportion to the driving current which drives said ultrasonic transducer,

c. A DC power source for supplying DC power to said oscillator,

d. converter means for providing a DC voltage in proportion to said driving current, a control circuit connected to said converter means for controlling DC power source, and

e. means for supplying liquid fuel to be atomized to the oscillating surface of said ultrasonic transducer;

B. an ignition electrode for igniting atomized liquid fuel,

C. a motor-driven blower for discharging atomized liquid fuel into a combustion chamber,

D. housing means surrounding an atomizing section including said ultrasonic transducer, said liquid fuel supply means, said ignition electrode and said motor-driven blower for causing combustion air to blow along said atomizing section,

B. photoelectric transducer means disposed within said housing means for providing a combustion signal in response to light emitted from the combustion flame in said combustion chamber,

F. means for controlling the supply of liquid fuel to said ultrasonic transducer, and

G. means for interrupting the operation of said DC power source, said ignition electrode, said motordriven blower and said liquid fuel supply control means in response to an absence of said combustion signal.

16. An ultrasonic liquid fuel burner as defined in claim 15 further comprising means for' detecting whether combustion air is flowing through said housing means, said interrupting means being adapted to be actuated in response to the signals from said air flow detecting means.

17. An ultrasonic liquid fuel burner as defined in claim 15 further comprising means for controlling said liquid fuel supply control means in response to said driving current.

18. An ultrasonic liquid fuel burner as defined in claim 15 further comprising timer means responsive to the interruption of power to said liquid fuel supply control means for initially delaying the interruption of power from said two-output DC power source to said motor-driven blower.

19. An ultrasonic liquid fuel burner as defined in claim 15 further comprising means for varying said DC voltage from said converter means in response to the temperature of liquid fuel to be atomized.

20. An ultrasonic liquid fuel burner as defined in claim 19 further comprising means for detecting whether combustion air is flowing through said housing means, and means for actuating said interrupting means in response to the signals from said air flow detecting means.

21. An ultrasonic liquid fuel burner as defined in claim 19 further comprising means for controlling said liquid fuel supply control means in response to said driving current.

22. An ultrasonic liquid fuel burner as defined in claim 19 further comprising timer means responsive to the interruption of power to said liquid fuel supply control means for initially delaying the interruption of power from said two-output DC power source to said motor-driven blower.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,885,902 Dated May 27, 1975 lnventofls) Hiroshi Fuj ieda, et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' In column 11, line 22: Change "claim 4" to claim 2-.

In column 12, line 39: Change "comrising" to comprising--.

Signed and Scaled this seventh Day of 0 c0 ber1975 [SEAL] Y A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner of Parents and Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,885,902 Dated May 27, 1975 lnventofls) Hiroshi Fuj ieda, et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In column 11, line 22: Change "claim 4" to -claim 2.

In column 12, line 39: Change "comrising" to comprising.

Signed and Scaled this seventh Of 0cfi0 ber1975 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner ofPatents and Trademarks 

1. An ultrasonic generator comprising in combination a. an ultrasonic transducer whose motional admittance becomes maximum at a resonant frequency, b. an oscillator comprising an amplifier driving said ultrasonic transducer with a square waveform voltage at a frequency substantially equal to said resonant frequency, whereby a surface of the transducer oscillates and a feedback circuit for feeding back to said amplifier an AC control voltage in proportion to the driving current which drives said ultrasonic transducer, c. a DC power source for supplying DC power to said oscillator, said DC power source being provided with a DC voltage regulator and d. means for converting said AC control voltage into a DC control input voltage for said regulator.
 2. An ultrasonic atomizer comprising an ultrasonic generator as Defined in claim 1 further comprising means for supplying liquid to be atomized to the oscillating surface of said ultrasonic transducer.
 3. An ultrasonic atomizer comprising an ultrasonic generator as defined in claim 1 wherein said DC power source comprising means for selectively supplying two different output voltages.
 4. An ultrasonic atomizer as defined in claim 2 wherein said converting means comprises a switching circuit for selectively supplying one of two different output voltages, and a control circuit for controlling said switching circuit.
 5. An ultrasonic atomizer as defined in claim 4 further comprising means responsive to said driving current for controlling the flow rate of liquid to be supplied to said ultrasonic transducer.
 6. An ultrasonic atomizer as defined in claim 4 further comprising means responsive to said driving current for controlling the flow rate of liquid to be supplied to said ultrasonic transducer.
 7. An ultrasonic atomizer as defined in claim 2 further comprising timer means connected to said DC voltage regulator for increasing said control input voltage from 0 percent to 100 percent within a predetermined time interval.
 8. An ultrasonic generator as defined in claim 7 further comprising means responsive to said driving current for controlling the flow rate of liquid to be supplied to said ultrasonic transducer.
 9. An ultrasonic atomizer as defined in claim 7 further comprising means for detecting the temperature of liquid to be atomized and for varying said control input voltage in response to the detected temperature.
 10. An ultrasonic atomizer comprising in combination a. an ultrasonic transducer whose motional admittance becomes maximum at a resonant frequency, b. an oscillator comprising an amplifier driving said ultrasonic transducer with a square waveform voltage at a frequency substantially equal to said resonant frequency, whereby the surface of the transducer oscillates and a feedback circuit for feeding back to said amplifier an AC control voltage in proportion to the driving current which drives said ultrasonic transducer, c. a DC power source supplying DC power to said oscillator, d. liquid to be atomized to the oscillating surface of said ultrasonic transducer and e. means responsive to said driving current for controlling the flow rate of liquid to be supplied to said ultrasonic transducer.
 11. An ultrasonic liquid fuel burner comprising A an ultrasonic generator comprising a. an ultrasonic transducer whose motional admittance is maximum at a resonant frequency, b. an oscillator comprising an amplifier adapted to drive said ultrasonic transducer with a square waveform voltage at a frequency substantially equal to said resonant frequency, and a feedback circuit for feeding back to said amplifier an AC voltage in proportion to the driving current which drives said ultrasonic transducer, c. A DC power source for supplying DC power to said oscillator, d. means for supplying liquid fuel to be atomized to an oscillating surface of said ultrasonic transducer, e. a DC power source providing two different output voltages, f. a switching circuit means for selecting one of said two output voltages of said DC power source, and g. a control circuit for controlling said switching circuit; B. an ignition electrode for igniting the atomized liquid fuel, C. a combustion chamber, D. a motor driven blower for discharging atomized liquid fuel, E. housing means surrounding an atomizing section including said ignition electrode and said motor-driven blower for causing air to flow along said atomizing section, F. photoelectric transducer means disposed within said housing means for providing a combustion signal in response to light emitted from the combustion flame in said combustion chamber, G. means for controlling the supply of liQuid fuel to be supplied to said ultrasonic transducer, and H. power supply control means connected to said DC power source, said ignition electrode, the motor for driving said blower and said liquid fuel supply control means for interrupting the operation of the apparatus connected thereto in response to an absence of the signal from said photoelectric transducer means.
 12. An ultrasonic liquid fuel burner as defined in claim 11 further comrising means for detecting whether air is flowing through said housing means, said power supply control means being adapted to be actuated in response to the signals from said air flow detecting means.
 13. An ultrasonic liquid fuel burner as defined in claim 11 further comprising means responsive to said driving current for controlling said liquid fuel supply control means.
 14. An ultrasonic liquid fuel burner as defined in claim 11 further comprising timer means for delaying the operation of said DC power source and said motor of said blower.
 15. An ultrasonic liquid fuel burner comprising A. an ultrasonic generator comprising a. an ultrasonic transducer whose motional admittance is maximum at a resonant frequency, b. an oscillator comprising an amplifier means for driving said ultrasonic transducer with a square waveform voltage at a frequency substantially equal to said resonant frequency whereby a surface of said transducer oscillates, and a feedback circuit for feeding back to said amplifier an AC voltage in proportion to the driving current which drives said ultrasonic transducer, c. A DC power source for supplying DC power to said oscillator, d. converter means for providing a DC voltage in proportion to said driving current, a control circuit connected to said converter means for controlling DC power source, and e. means for supplying liquid fuel to be atomized to the oscillating surface of said ultrasonic transducer; B. an ignition electrode for igniting atomized liquid fuel, C. a motor-driven blower for discharging atomized liquid fuel into a combustion chamber, D. housing means surrounding an atomizing section including said ultrasonic transducer, said liquid fuel supply means, said ignition electrode and said motor-driven blower for causing combustion air to blow along said atomizing section, E. photoelectric transducer means disposed within said housing means for providing a combustion signal in response to light emitted from the combustion flame in said combustion chamber, F. means for controlling the supply of liquid fuel to said ultrasonic transducer, and G. means for interrupting the operation of said DC power source, said ignition electrode, said motor-driven blower and said liquid fuel supply control means in response to an absence of said combustion signal.
 16. An ultrasonic liquid fuel burner as defined in claim 15 further comprising means for detecting whether combustion air is flowing through said housing means, said interrupting means being adapted to be actuated in response to the signals from said air flow detecting means.
 17. An ultrasonic liquid fuel burner as defined in claim 15 further comprising means for controlling said liquid fuel supply control means in response to said driving current.
 18. An ultrasonic liquid fuel burner as defined in claim 15 further comprising timer means responsive to the interruption of power to said liquid fuel supply control means for initially delaying the interruption of power from said two-output DC power source to said motor-driven blower.
 19. An ultrasonic liquid fuel burner as defined in claim 15 further comprising means for varying said DC voltage from said converter means in response to the temperature of liquid fuel to be atomized.
 20. An ultrasonic liquid fuel burner as defined in claim 19 further comprising means for detecting whether combustion air is flowing through said housing means, and means for actuating said interrupting Means in response to the signals from said air flow detecting means.
 21. An ultrasonic liquid fuel burner as defined in claim 19 further comprising means for controlling said liquid fuel supply control means in response to said driving current.
 22. An ultrasonic liquid fuel burner as defined in claim 19 further comprising timer means responsive to the interruption of power to said liquid fuel supply control means for initially delaying the interruption of power from said two-output DC power source to said motor-driven blower. 